(a) Field of the Invention
The present invention relates to a system and method for synchronizing individual units of a wireless network using the time information of synchronous mobile terminals. More specifically, the present invention relates to a system and method for providing NTP (Network Time Protocol) stratum-1 servers through a Bluetooth interface using IS-95/2000 mobile terminals providing a precise and stable time, as a reference clock.
(b) Description of the Related Art
The existing computer clocks have a limitation of accuracy and precision because of their inherent instability, environmental factors, users' random adjustments, or the like. There is thus a great need for a global time standard unique to distributed systems. Namely, network time synchronization is necessary. Network time synchronization for the Internet or the like is provided in various forms according to the adapted algorithm or the characteristic of the synchronization structure concerned, but basically the clients requesting time synchronization read the time of the server that provides time synchronization. An example is the network time protocol (NTP) by Mills that is currently adopted as an Internet standard.
The NTP is a system for time synchronization of computer clocks on the Internet developed in the Delaware University in the U.S.A, and it is used for time synchronization between a distributed time server (DTS) and clients. The NTP-based time synchronization method is an application of the phase-lock method in which the clock is synchronized in a reliable error range by communication of time stamp messages among servers or with a plurality of subnet peers. The NTP version 1 was first described in RFC-958, and it has evolved to the NTP version 2 in RFC-1119. Currently, the RFC-1305 describes the definition for the NTP version 3.
Next, a description will be given as to the schematic configuration of an NTP processor with reference to FIG. 5.
Time synchronization information is transferred from a plurality of peers through a remote processor 800. First, second, and third filters in a peer processor 810 reduce incidental time errors of the delay and clock differences (clock offset) among the systems to be synchronized. A system processor 820 selects one of the time stamps that has the shortest transmission delay, and transfers the offset of the corresponding time stamp to a selection algorithm. A clock control processor 830 selects the most precise and reliable time from the time stamps received from the multiple servers using the clock selection algorithm, and adopts the selected time as a reference.
Next, a description will be given as to the NTP stratum in brief. The NTP has a hierarchical topology that analyzes the time stamping packets received in response to the service request to at least upper layers (upper servers). The uppermost NTP server that is directly synchronized to the atomic clock or international standard time is called an “NTP Stratum-1 server.” A server that is synchronized to the time received from the NTP stratum-1 servers and that operates as an NTP server for the lower system is called an “NTP Stratum-2 server.” The stratum of the NTP is configured in this manner such that the stratum number is assigned to the NTP according to the sequence number of the stratum.
According to the sensors in 2000, there are at the time of writing about 100 NTP primary servers located in North America, Europe, and the Pacific, almost one third of which are advertised for public access. These public servers are synchronized to national time standards using all known computer-readable time-dissemination services in the world, including the U.S. (WWWVB, WWV, and WWVH), Canada (CHU), the U.K. (MSF), Germany (DCF77), and France (TDF), as well as the GPS, OMEGA, and LORAN navigation systems, and the GOES (Geosynchronous Orbiting Environmental Satellite). In addition, NTP primary servers at the national time standards laboratories of America, Norway, and Australia are directly synchronized to national standard clock ensembles. Some NTP primary servers are located at the Korea Research Institute of Standards and Science.
As described above, NTP servers can be used to synchronize the nodes of a network to one another. Here, the NTP servers must be synchronized to an accurate clock. Conventionally, the NTP servers contain an atomic clock (e.g., cesium or rubidium clock) and are directly synchronized to the GPS.
But atomic clocks are expensive, and the GPS reception is difficult indoors. Most electrical home appliances, for example PCs of a network, contain a local clock with an oscillator comprised of inexpensive quartz. The frequency cycle of these oscillators is readily changed by environmental effects such as electronic noise or heat. This instability increases the error of the individual local clocks with the passage of time. The simplest method of enhancing the stability is to replace the ordinary quartz clock with an expensive atomic clock or a GPS clock, which method is troublesome with a substantial expense.